1. Field of the Invention
The disclosed invention involves relates to optical waveguides, and more particularly to, optical waveguides useful for sensing temperature and pressure changes by monitoring the effects on coupling between transmission modes in the optical waveguide core.
2. Description of Related Art
The usefulness of distributed fiber-optic systems is well known in the art. These systems are formed by combining fiber sensing and fiber telemetry features. Typical approaches to the distributed system may be categorized as being either intrinsic distributed or quasi-distributed. Intrinsic distributed sensing uses a single length of fiber to form an extended sensor element which senses the measureand field continuously over its entire length. Quasi-distributed sensing uses discrete sensor elements arranged in a linear array or some other useful network topology.
Intrinsic distributed sensors are particularly attractive for applications where monitoring of a single measureand (such as temperature or hydrostatic pressure) is required at a large number of points or continuously over the path of the fiber. Application examples include stress monitoring for real-time evaluation of structural integrity of mobile platforms or buildings; temperature profiling in electrical power transformers, transmission lines and generators; and simple intrusion detection systems. Intrinsic distributed sensing is particularly useful for development of "intelligent" structures by embedding continuous sensors in "smart" patterns within composite materials.
Quasi-distributed sensors and sensor networks are useful for monitoring single measureands but may be more useful for multi-measureand monitoring by selecting different sensors for different measureands within an array. Applications include process control, and mobile platform performance monitoring. Quasi-distributed sensor networks are not limited to the use of fiber-optics for sensors and may combine them with other sensors in a discrete arrangement.
Both intrinsic distributed and quasi-distributed fiber optic sensor systems require either distributed sensors or individual sensor elements together with scanning and addressing techniques. Intrinsic distributed sensors typically use either basic loss or scattering mechanisms to sense a measureand in a single length of optical fiber, which forms an extended sensor. The most basic form of an intrinsic-distributed sensor relies on the detection of regions of localized increases in loss, caused by microbending in a length of fiber, by direct Optical Time-Domain Reflectometry (OTDR) analysis of the Rayleigh-backscattered light. This is useful as a disturbance location sensor for simple intrusion detection systems. Another such sensor uses the change in the Rayleigh-scattering coefficient with temperature to perform distributed temperature sensing. In general, this dependance is extremely weak in solid core fibers.
One method known in the art for improving the sensitivity of optical fibers to changes in temperature and stress uses an optical fiber comprising two or more cores having refractive indices different from the cladding material. The exact spacing separating the cores, the number of cores, and the refractive prefiles are critical to the optimum performance of such a sensor. These parameters are critical because the sensing function relies on the optical cross-coupling between the internal cores in the fiber. Fabrication of such multicore fibers to tight tolerances is very difficult.
U.S. Pat. No. 4,295,738 issued Oct. 20, 1981 to Gerald Meltz, et al discloses an optical fiber, having at least two cores positioned in a common cladding, fabricated to be responsive to strain or hydrostatic pressure (but not to temperature) through the selection of materials, spacing and shape of the cores and cladding of the fiber. U.S. Pat. No. 4,295,739 issued Oct. 20, 1981 to Gerald Meltz, et al discloses the same multi-core optical fiber, with a plurality of cores positioned across the diameter of a common cladding, optimized to respond to either temperature or strain by the selection of materials, spacing and the shaping of the cores in the fiber. Meltz, et al teaches the use of cores, having cylindrical or elliptical cross-section, embedded in a common cladding so that the optical cross-talk between cores varies as a known function of fiber temperature and pressure. The fiber and core dimensions are selected so that light propagation in each core is limited to the lowest order He.sub.11 mode in a manner well-known in the art.
The Meltz, et al invention suffers from two problems associated with limitations of the fabrication process. First, the coupling between fibers having a circular or elliptical cross-section is very sensitive to the exact cross-sectional dimensions and relative location of the cores. This means that the internal core dimensions within the fiber must be controlled both in position and shape to very tight tolerances over the length of the optical fiber sensor, which may be substantial. Errors in position and shape act to severely reduce distributed sensor performance. Second, the degree of cross-coupling between cylindrical or elliptical cores is limited by the curved shape of a core and the relatively large distance between the cores. The Meltz, et al invention is unavoidably limited in performance and sensitivity because of these limitations on core cross-coupling. This cross-coupling can be significantly increased to improve sensitivity only by making the cores strongly elliptical and reducing the distance between adjacent cores to a very low value. The difficulty in maintaining necessary fabrication tolerances forces the manufacturer to limit the core ellipticity and separation, which limits the available performance sensitivity of such multi-core fibers.
U.S. Pat. No. 4,461,536 issued Jul. 24, 1984 to Herb J. Shaw, et al discloses a transducer for use as an accurate sensor of physical parameters such as temperature and pressure. Shaw, et al overcomes the above limitations on cross-coupling, but he does so by using two fibers which are shaved and placed together in a jig to sense minute displacements. This displacement sensing capacity is then used to sense temperature and pressure from secondary sensors serving to convert temperature and pressure into minute displacements. Shaw, et al does not suggest that his method of shaving two single-core fibers and placing them together in a jig has any application to distributed fiber-optic sensors of significant length.
As noted above, distributed fiber optic sensors are known in the art to be useful for detecting structural flaws in composite structures by positioning sensing fibers within the structure during manufacture and examining the sensor output to detect structural failure. U.S. Pat. No. 3,910,105 issued Oct. 7, 1975 to Donald A. Hoffstedt discloses such a method as does U.S. Pat. No. 4,836,030 issued Jun. 6, 1989 to David A. Martin. The prospects of applying distributed fiber optic stress and temperature sensors to the real-time monitoring of composite structures is a strong incentive for development of very long fiber-optic distributed sensors having sensitive and reliable capability for measuring temperature and stress.
The compatibility of optical fiber sensors with composite materials permits installation of internal monitoring devices prior to curing of the composite. Proper installation of the optical fiber sensor within the laminate should provide a void-free component with minimal disturbance to the pattern of the reinforcement filaments. Such an installation provides quantitative real-time measures of the stress conditions in the composite structure.
It is known in the art that intrinsic-distributed sensor systems can be fabricated using one of two different types of dual-mode optical fibers. The first type is the twin-core fiber discussed above, and the second type is the dual-mode birefringent optical fiber. In both cases measurements are conducted on the differential phase shifts between two modes. This relative phase change appears as a cross-talk change in the twin core fiber and a polarization change in the birefringent fiber. As known in the art, operation at multiple wavelengths serves to resolve two parameters. For instance, the twin-core fiber can measure temperature and strain simultaneously, while the birefringent fiber can measure the strains necessary for a two-dimensional stress analysis.
The twin-core fiber sensor is quite different from the birefringent device. The twin-core fiber has two matched single-mode cores which are very closely spaced. When one core is illuminated, both symmetric and antisymmetric modes of the fiber are equally excited and light couples back and forth between the cores as it propagates along the fiber. Complete power transfer takes place over a distance defined as the beat length of the dual mode interference pattern which is linearly distributed along the sensor. The variation in light intensity in each core is a periodic function of the beat. In general, both the beat length and beat phase vary with temperature and strain. Temperature and strain can be measured simultaneously by detecting the response in the core contrast ratio at two wavelengths.